docs: def helical_laminar_fd_White(Re, Di, Dc):
      r'''Calculates Darcy friction factor for a fluid flowing inside a curved 
      pipe such as a helical coil under laminar conditions, using the method of 
      White [1]_ as shown in [2]_.
      
      .. math::
          f_{curved} = f_{\text{straight,laminar}} \left[1 - \left(1-\left(
          \frac{11.6}{De}\right)^{0.45}\right)^{\frac{1}{0.45}}\right]^{-1}
  
      Parameters
      ----------
      Re : float
          Reynolds number with `D=Di`, [-]
      Di : float
          Inner diameter of the coil, [m]
      Dc : float
          Diameter of the helix/coil measured from the center of the tube on one
          side to the center of the tube on the other side, [m]
  
      Returns
      -------
      fd : float
          Darcy friction factor for a curved pipe [-]
  
      Notes
      -----
      The range of validity of this equation is :math:`11.6< De < 2000`,
      :math:`3.878\times 10^{-4}<D_i/D_c < 0.066`.
      
      The form of the equation means it yields nonsense results for De < 11.6;
      at De < 11.6, the equation is modified to return the straight pipe value.  
      
      This model is recommended in [3]_, with a slight modification for Dean 
      numbers larger than 2000.
  
      Examples
      --------
      >>> helical_laminar_fd_White(250, .02, .1)
      0.4063281817830202
  
      References
      ----------
      .. [1] White, C. M. "Streamline Flow through Curved Pipes." Proceedings of
         the Royal Society of London A: Mathematical, Physical and Engineering 
         Sciences 123, no. 792 (April 6, 1929): 645-63. 
         doi:10.1098/rspa.1929.0089. 
      .. [2] El-Genk, Mohamed S., and Timothy M. Schriener. "A Review and 
         Correlations for Convection Heat Transfer and Pressure Losses in 
         Toroidal and Helically Coiled Tubes." Heat Transfer Engineering 0, no. 0
         (June 7, 2016): 1-28. doi:10.1080/01457632.2016.1194693.
      .. [3] Blevins, Robert D. Applied Fluid Dynamics Handbook. New York, N.Y.: 
         Van Nostrand Reinhold Co., 1984.
      '''
      De = Dean(Re=Re, Di=Di, D=Dc)
      fd = friction_laminar(Re)
      if De < 11.6:
          return fd
      return fd/(1. - (1. - (11.6/De)**0.45)**(1./0.45))